Thomsen_2022_N.Biotechnol__

This work examines the significance of the degree of polyethylene terephthalate (PET) crystallinity (X(C)) and glass transition temperature (T(g)) on enzymatic degradation of PET at elevated temperatures using two engineered, thermostable PET degrading enzymes: LCC(ICCG), a variant of the leaf-branch compost cutinase, and DuraPETase, evolved from the Ideonella sakaiensis PETase. X(C) was systematically varied by thermal annealing of PET disks ( 6mm, thickness 1mm). X(C) affected the enzymatic product release rate that essentially ceased at X(C) 22-27% for the LCC(ICCG) and at X(C) -17% for the DuraPETase. Scanning Electron Microscopy revealed that enzymatic treatment produced cavities on the PET surface when X(C) was >10% but resulted in a smooth surface on amorphous PET (X(C) -10%). The T(g) of amorphous PET disks decreased from 74 degreesC to 61 degreesC during 24h pre-soaking in water at 65 degreesC, while X(C) remained unchanged. Enzymatic reaction on pre-soaked disks at 65 degreesC, i.e. above the T(g), did not affect the enzymatic product release rate, but delayed the initiation of enzymatic attack despite the lower T(g) compared to enzymatic reaction on un-soaked samples. The data suggest that extended soaking of PET at 65 degreesC induces an increase in the rigid amorphous fraction (X(RAF)) that impedes the enzymatic attack. These findings improve the understanding of enzymatic PET degradation and have implications for development of efficient enzymatic PET upcycling processes.